This invention relates to a high-density rewritable optical recording medium and, more particularly, to a phase-change type recording medium having excellent performance in reduced recording signal jitter, wide write power margin, durability against characteristics deterioration by repeated overwriting, archival stability, and the like.
Media which are rewritable and interchangeable with compact discs, such as CD-RWs, have already spread. Media rewritable and interchangeable with DVDs, such as DVD-RWs, DVD+RWs, and DVD-RAMs, have now been prevailing. These phase-change optical discs are inexpensive large-capacity recording media excellent in portability, weatherability, impact resistance and the like.
The phase-change optical recording media accomplish writing and erasing by making use of changes in reflectance with reversible changes of the crystalline state. In general, a crystalline state is used as a non-recorded or erased state, where amorphous marks are formed for writing. A recording layer is usually crystallized by heating and keeping around the crystallization temperature for a given time and made into an amorphous phase by heating to a temperature higher than the melting point followed by quenching. As is understood from the difference in heating temperature, a crystalline phase is generally more stable.
Chalcogen alloys, such as GeSbTe alloys, InSbTe alloys, GeSnTe alloys, and AgInSbTe alloys, are of frequent use as a recording layer. These alloys are materials capable of overwriting.
In particular pseudo-binary alloys composed of Sb2Te3 and GeTe (hereinafter simply referred to as pseudo-binary alloys) and alloys having a composition near to an Sb70Te30 eutectic composition as a main component are known.
They are stable in both the crystalline state and the amorphous state and show phase transition between the two states relatively rapidly. They also have a advantage that segregation hardly occurs in repetition of overwriting. Accordingly, they have been put to practical use as a recording layer of phase-change optical discs.
The term xe2x80x9coverwrite (direct overwrite)xe2x80x9d denotes a recording mode in which when information is recorded on a medium having information already recorded information is recorded thereon without erasing the existing information before recording, i.e., a mode of recording while erasing the existing information. Since phase-change media usually accomplish recording in an overwrite mode, xe2x80x9coverwritexe2x80x9d can be simply xe2x80x9crecordxe2x80x9d.
Of the alloys having a composition near to the Sb70Te30 eutectic composition as a main component, those containing Sb in excess over the Sb70Te30 eutectic composition (hereinafter simply referred to as an eutectic alloy systems) are attracting attention recently. A recording layer made of this type of alloys forms amorphous marks with smooth edges (mark edges) which are effective in suppressing jitter, and shows an extremely high crystallization rate so that it is capable of high-speed overwriting.
A phase-change recording medium is in general a so-called high-to-low medium, which has a lower reflectance after recording than before recording. Because a crystalline state is usually utilized as a non-recorded or erased state, and an amorphous state as a recorded state, the recording layer has a lower reflectance in an amorphous state than in a crystalline state. A high-to-low medium typically has a layer structure composed of a first protective layer, a recording layer, a second protective layer, and a reflective layer.
On the other hand, so-called low-to-high media which have a higher reflectance after recording than before recording are also known. Recording media of this type have a high reflectance when the recording layer is in an amorphous state than when the recording layer is in a crystalline state.
As for the low-to-high media, a layer structure composed of a semi-transparent layer mainly comprising metal, a first protective layer having a dielectric substance, a recording layer, a second protective layer having a dielectric substance, and a metallic reflective layer in this order is known. It is generally said that the low-to-high media having such a layer structure are capable of reducing a cross-erase phenomenon as compared with the high-to-low media.
Utility of the low-to-high media in media having a pseudo-binary alloy system recording layer is known. For example, where an alloy near to Ge2Sb2Te5, which is a typical example of the pseudo-binary alloy system, is used as a recording layer, it is accepted that erasing unevenness due to difference in light absorption between amorphous areas and crystalline areas is eliminated to make high-speed overwriting possible.
However, utility of the low-to-high media having the above-described layer structure in media having an eutectic alloy system recording layer has not been made clear.
There have not been data indicating that application to the low-to-high media is effective on reduction of cross-erase. Since the eutectic alloy system recording layer essentially enables high-speed overwriting, contribution to the low-to-high media in this respect is not clear, either.
It has rather been observed that application to the low-to-high media can result in considerable reductions in archival stability or repeated overwrite characteristics. While some reduction of repeated overwrite characteristics is also observed with the high-to-low media, the degree of reduction occurring in the low-to-high media is higher.
That is, a general conventional layer structure for the low-to-high media has not succeeded in attaining excellent characteristics when using an eutectic alloy system recording layer.
The inventors have extensively studied on the problems associated with low-to-high media having an eutectic alloy system recording layer. As a result, they have found that a specific combination of layer structure design and material selection for each layer makes it possible for the first time to provide a medium with excellent characteristics and proved this medium superior to general high-to-low media in repeated overwrite characteristics or write power margin. The present invention has been completed based on these findings.
The gist of the present invention consists in an optical recording medium having a semi-transparent layer mainly comprising Ag, a first protective layer comprising a dielectric, a phase-change recording layer made of an alloy mainly comprising SbxTe1xe2x88x92x (0.7 less than xxe2x89xa60.9), a second protective layer comprising a dielectric, and a metallic reflective layer in this order, which is characterized in that, where the first protective layer comprises a sulfur-containing dielectric, the medium further has an interdiffusion-protection layer between the semi-transparent layer and the first protective layer and that the medium has a higher reflectance to light incident on the semi-transparent layer side with the recording layer being in an amorphous state than with the recording layer being in a crystalline state.
Another gist of the present invention consists in an optical recording medium having a semi-transparent layer mainly comprising Ag, a first protective layer comprising a dielectric, a phase-change recording layer, a second protective layer comprising a dielectric, and a metallic reflective layer in this order, which is characterized in that crystallization in the recording layer proceeds chiefly through crystalline growth from the boundaries between amorphous or molten areas and crystalline areas, that the medium further has an interdiffusion-protection layer between the semi-transparent layer and the first protective layer where the first protective layer comprises sulfur, and that the medium has a higher reflectance to light incident on the semi-transparent layer side with the recording layer being in an amorphous state than with the recording layer being in a crystalline state.